The Mayo Clinic on recently (Aug. 15, 2012) announced that Rep. Jesse Jackson Jr. has bipolar disorder, the clearest indication so far of what’s kept the veteran South Side lawmaker on medical leave and out of the public eye for more than two months.

But Jackson’s family, staff members and the clinic would not discuss a timetable on the congressman’s return to work or specifics of his treatment. Jackson, 47, is up for re-election Nov. 6.

The official diagnosis follows weeks of evolving information on the Democratic congressman’s condition, which was first described as a “exhaustion,” then “physical and emotional ailments” and later a “mood disorder.” When Jackson transferred from an Arizona treatment facility to Mayo in Rochester, Minn., in late July, the clinic cited “depression and gastro-intestinal issues.”

On Monday, medical officials said Jackson has bipolar II depression, the milder form of a mental illness once referred to as manic depression. The more severe form of bipolar disorder affects about 1 to 2 percent of the population. That percentage goes up — to about 5 percent — if you count people with the milder form and those who show some symptoms of bipolar disorder but do not fit the diagnostic criteria.

The condition is diagnosed as either bipolar I or II disorder. The second type is considered the milder form.

Those with the more severe form can have episodes of mania — excessive agitation or extreme euphoria — that last for long periods, said Dr. Daniel Yohanna, vice chairman of the department of psychiatry and behavioral neuroscience at University of Chicago Medicine. In some cases the person can become psychotic. Those episodes often result in hospitalization.

With the second type — the type that Jackson has — the person might experience similar but less severe symptoms, such as being overly talkative, having feelings of grandiosity and not feeling the need for sleep, Yohanna said. Those episodes are referred to as hypomania.

People with either type of the disorder experience depression, which can be debilitating at its worst. They typically alternate among periods of normal mood, depression and either mania or hypomania.

Those who are experiencing hypomania can “feel good, energetic and productive and may not know there’s a problem,” said Dr. Michael Ziffra, assistant professor of psychiatry at Northwestern University’s Feinberg School of Medicine. He said hypomania can lead to risky behaviors, such as sexual promiscuity, excessive spending and fighting.

“What’s happening in the brain with bipolar disorder is more complicated than with regular major depression,” Ziffra said. “With depression, we have identified certain neurotransmitters that definitely seem to be playing a role. With bipolar disorder, it appears to be more complex. You can’t say it’s one neurotransmitter or region of the brain.”

Doctors believe that people who have the illness are born with a genetic predisposition for it. But environmental and other factors also appear to play a role.

“It certainly has a genetic component,” Yohanna said. “But that by no means accounts for everyone who gets it. Genetics accounts for only about 30 to 40 percent of the variability of getting the disease, so it isn’t completely genetic. There are other environmental and other factors we don’t understand.”

Most people become symptomatic during their teens or 20s, Ziffra said. Some people start having symptoms even earlier, but it’s not unusual to be diagnosed later in adulthood, he said.

With the less severe type, it is possible to go many years not seeking treatment, or seeking treatment and not being diagnosed properly, he said.

If Jackson’s disorder did not become serious until his 40s, it might suggest that it’s a less severe form that will respond well to treatment, said Dr. Johnny Williamson, a psychiatrist at Community Counseling Centers of Chicago.

“The most severe forms usually start much earlier in life,” he said. “To reach this level of life before he was diagnosed may be a positive indicator.”

The Mayo statement Monday said that “Jackson is responding well to the treatment and regaining his strength.”

Treatment for bipolar disorder involves psychotherapy — “talk therapy” — and medication such as Depakote or lithium, both of which fall into a class of drugs called mood stabilizers, Yohanna said. Medicines called antipsychotics also can be prescribed to stabilize mood, Ziffra said.

Most people with the type that Jackson has function well after being treated, Ziffra said.

The Mayo statement noted that Jackson underwent weight loss surgery, a duodenal switch, in 2004, and that such surgery “can change how the body absorbs food, liquids, vitamins, nutrients and medications.” Mayo spokeswoman Traci Klein declined to comment on whether medical professionals believe there is a link between Jackson’s gastrointestinal surgery and his bipolar condition.